The present invention relates to a sliding member formed by adhering a coating layer composed of solid lubricant plate crystal particles to the surface of a base material, and a method for forming the coating layer.
In the slide bearing (sliding member) of an automotive engine, generally, an aluminum-base bearing alloy and a copper-base bearing alloy are used as the sliding layer. In recent years, while advances have been made which permit higher output power and higher rotational speed in automotive engines, the demand for improvement of fuel efficiency is high, and there is therefore a need for a low friction characteristic in the slide bearing. In particular, in vehicles, like a hybrid car, where frequent starting and stopping of an engine are repeated, a low friction characteristic under boundary lubricant conditions is required.
In order to address the demand for a low friction characteristic under boundary lubricant conditions, reforming the surface of a slide bearing by using a solid lubricant is effective. As to the reforming of the surface of a bearing by means of a solid lubricant, there are known a technique (Patent Documents 1 to 3), in which solid lubricant particles are mixed with a resin binder and then coated onto the surface of the slide bearing and cured, and a technique (Patent Documents 4 to 6) in which by applying a technique, such as shot peening or shot blasting, solid lubricant particles are collided against the surface of the slide bearing and adhered to the surface of the slide bearing by the collision energy.
The Patent Document 1 describes a slide bearing material wherein a coating layer composed of a solid lubricant of 55 to 90 mass % and a polyimide resin binder of 45 to 10 mass % is formed in the surface of an aluminum-base bearing alloy, and states that this coating layer improves the initial compatibility of the aluminum-base bearing alloy, and thus an excellent fatigue resistance and an excellent anti-seizure property are obtained. Patent Document 2 describes that a coating layer composed of a solid lubricant of 30 to 90 mass % and a resin binder of 70 to 10 mass % is formed in the surface of a copper system bearing alloy, thereby attaining improvement in fatigue resistance, wear resistance, and anti-seizure property. Moreover, Patent Document 3 describes that a coating layer composed of a solid lubricant of 30 to 70 mass % and a resin binder of 70 to 30 mass % is formed in the surface of a bearing base material, thereby obtaining an excellent wear resistance and an excellent sliding characteristic.
On the other hand, Patent Document 4 describes a lubricant coated bearing wherein solid lubricant particles are collided against the surface of a bearing with a sufficient energy by the same method as that of sand blasting, thereby forming a film in the thickness of 75 to 100 μm. Patent Document 5 describes that solid lubricant particles are injected onto the surface of a sliding portion composed of metal or ceramics at an injection velocity of 80 m/s or more, or at an injection pressure of 0.3 MPa or more, thereby diffusing and infiltrating the elements in the composition of the lubricant particle and forming a wear resistant film. Moreover, Patent Document 6 describes molybdenum disulfide with a purity of 95% or more and the mean particle diameter of 1 μm to 20 μm is collided against a piston of an engine, a cylinder bore, or the like at an injection velocity of 100 m/S or more, thereby forming a film, and states that an excellent lowering effect of the coefficient of friction is obtained due to this film.
Patent Document 1 JP-A-4-83914
Patent Document 2 JP-A-9-79262
Patent Document 3 JP-A-11-106779
Patent Document 4 U.S. Pat. No. 3,632,368
Patent Document 5 Japanese Patent No. 3,357,586
Patent Document 6 JP-A-2002-339083
As described above, techniques of fixing a solid lubricant to the surface of a base material of a sliding member include a technique of mixing solid lubricant particles with a resin binder and coating, and a technique of colliding solid lubricant particles against the surface of a base material and adhering.
However, because the one formed by coating uses a resin binder, the heat conductivity of the coating layer is low and the frictional heat generated by the slide of the counter member is hard to escape to the housing side, resulting in a low heat conduction. This causes a problem that seizure is likely to occur especially under starved lubricant conditions.
In the technique of forming a coating layer composed of a solid lubricant by colliding solid lubricant particles against the surface of a base material, the surface of the base material is hardened by crash impact of the solid lubricant particles, thereby inviting a decrease in compatibility and lowering the anti-seizure property. If attempting to maintain a soft surface state of the base material, the collision velocity of solid lubricant particles needs to be reduced, which would however worsen the adherability of the solid lubricant particles, and as a result, a required thickness is not obtained and a sufficient anti-seizure property cannot be obtained.
As described above, conventionally, when reforming the surface of the base material of a sliding member by means of a solid lubricant, a poor thermal conductive resin binder is used or a strain hardening of the surface of the base material is involved, which is problematic since it is difficult to obtain a sufficient anti-seizure property.
The present invention has been made in view of the above situation, and it is a first object of the present invention to provide a sliding member for reforming the surface of a base material by using a solid lubricant, the sliding member capable of obtaining an excellent anti-seizure property even under starved lubricant conditions, and it is a second object of the present invention to provide a method for forming a coating layer of a sliding member, the method capable of forming a coating layer composed of a solid lubricant in the surface of the base material without applying a resin binder to the surface of the base material of the sliding member and moreover without strain hardening the surface of the base material.
Molybdenum disulfide, graphite, tungsten disulfide, and boron nitride are often used as the solid lubricant. These have a form of a plate crystal particle, and the plate crystal particle has a lamellar crystal structure in which layers whose largest plane is primarily the (001) plane in Miller index are stacked in parallel. For example, the plate crystal particle of molybdenum disulfide has a structure in which the layers of MoS2 molecules connected in the direction parallel to the x-y plane are stacked in the z axis, as shown in FIG. 5, and only a weak van der Waals force is acting between adjoining layers 10 and 12.
If a plate crystal particle of such lamellar crystal structure is present as the solid lubricant in the coating layer of a sliding member, and a shearing force acts between layers in the plate crystal particles along with the movement of the counter member, it will overcome the van der Waals force easily, causing a slide between the layers. This slide between the layers is specific to the material with a lamellar crystal structure, and the coefficient of friction is extremely low. This is a mechanism by which the sliding member has a low friction when plate crystal particles are used as the solid lubricant.
Because the mechanism of a low friction characteristic of the solid lubricant plate crystal particles having a lamellar crystal structure is the one described above, the present inventor predicted that if a plane between layers in the solid lubricant plate crystal particles forming the coating layer is in parallel with the moving direction of the counter member, i.e., in parallel with the surface of the coating layer, a further low friction may be achieved, and has thus made the present invention.